Method for reducing dark current in image sensor

ABSTRACT

A method for fabricating a CMOS image sensor having a characteristic of a reduced dark current includes the steps of: a) providing a semiconductor structure, wherein the semiconductor structure includes a photodiode and peripheral elements formed on a semiconductor substrate; b) forming an insulating layer on the semiconductor structure; c) forming a hydrogen containing dielectric layer on the insulting layer; d) diffusing hydrogen ions contained in the hydrogen containing dielectric layer into a surface of the photodiode, thereby removing a dangling bond; and e) removing the hydrogen containing dielectric layer.

FIELD OF THE INVENTION

[0001] The present invention relates to a semiconductor device; moreparticularly, to a method for fabricating a CMOS image sensor having aplurality of unit pixels which are capable of reducing a dark current.

DESCRIPTION OF THE PRIOR ART

[0002] As is well known, an image sensor is an apparatus for sensing alight beam reflected from an object to generate an image data.Especially, an image sensor fabricated by using a complementary metaloxide semiconductor (CMOS) technology is called a CMOS image sensor.

[0003] Generally, the CMOS image sensor includes a plurality of unitpixels. Each of the unit pixels also includes a light sensing elementand a plurality of transistors. The light sensing element such as aphotodiode senses incident light beam to generate photoelectric chargescorresponding to an amount of the incident light beam. The transistorsperform switching operations to control a transfer of the photoelectriccharges.

[0004]FIG. 1 is a cross-sectional view showing sequential steps offabricating a conventional unit pixel contained in a CMOS image sensor.

[0005] Referring to FIG. 1, a P-type well 12 and a field oxide layer 13are formed on a semiconductor substrate 11, and a PN junction region 17Aand 17B is formed in the semiconductor substrate 11 to thereby provide aphotodiode 17. Then, a floating junction region 18A, to whichphotoelectric charges generated in the photodiode 17 is transferred, isformed within the semiconductor substrate 11.

[0006] Then, a transfer transistor TX for transmitting the photodiode tothe floating junction region 18A and a reset transistor RX for resettingthe floating junction region 18A are formed on the semiconductorsubstrate 11. A drive transistor DX for amplifying a voltage levelcorresponding to the transferred photoelectric charges and a selecttransistor SX for outputting amplified voltage level as the image dataare formed on the P-type well 12. At this time, the reset transistor RXand the drive transistor SX are commonly coupled to a common junctionregion 18B, and an impurity junction region 19 of a lightly loped drain(DLL) structure is formed between the drive transistor DX and the selecttransistor SX. Also, spacers 20 are formed on sidewalls of eachtransistors TX, RX, DX and SX.

[0007] Then, pre-metal dielectric (PMD) layers 21 and 22 are formed onthe transistors TX, RX, DX and SX, and interlayer insulating layers 23,24 and 25 are formed on the PMD layer 22.

[0008] Then, the PMD layers 21 and 22 and the interlayer insulatinglayers 23, 24 and 25 are selectively etched and first metal lines M1, M2and M3 and a second metal line M4 are formed thereon, respectively. Thefirst metal lines M1, M2 and m3 and the second metal line M4 used toconnect the transistors TX, RX, DX and SX with an external elements areformed with staked layers of Ti/Al/TiN.

[0009] Then, a passivation layer formed with an oxide layer 29 and anitride layer 30 is formed on the second metal line M4.

[0010] Then, a color filter array (CFA) operation is carried out tothereby form a color filter 31, and a dyed photoresist 32 is formed onan entire structure. Then, a microlens 33 is formed above a portionwhere the color filter 31 is formed.

[0011] At this time, a number of surface energy states exist in aforbidden band due to a dangling bond of a lattice structure. Thesurface energy states result in a recombination of carriers. As aresult, a leakage current is increased and a breakdown voltage of theimage sensor may be influenced. That is, an undesired dark current isflowed so that a reliability of the image sensor is degraded.

SUMMARY OF THE INVENTION

[0012] It is, therefore, an object of the present invention to provide amethod for fabricating a CMOS image sensor having a unit pixel which iscapable of reducing a dark current.

[0013] In accordance with an aspect of the present invention, there isprovided a method for fabricating a CMOS image sensor, wherein the CMOSimage sensor includes a plurality of unit pixels, the method comprisingthe steps of: a) providing a semiconductor structure, wherein thesemiconductor structure includes a photodiode and peripheral elementsformed on a semiconductor substrate; b) forming an insulating layer onthe semiconductor structure; c) forming a hydrogen containing dielectriclayer on the insulting layer; d) diffusing hydrogen ions contained inthe hydrogen containing dielectric layer into a surface of thephotodiode, thereby removing a dangling bond; and e) removing thehydrogen containing dielectric layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Other objects and aspects of the invention will become apparentfrom the following description of the embodiments with reference to theaccompanying drawings, in which:

[0015]FIG. 1 is a cross-sectional view showing sequential steps offabricating a conventional CMOS image sensor; and

[0016]FIGS. 2A, 2B, 2C, 2D, 2E and 2F are cross-sectional viewsillustrating sequential steps of fabricating a CMOS image sensor inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017]FIGS. 2A to 2F are cross-sectional views illustrating sequentialsteps of fabricating a unit pixel in accordance with the presentinvention.

[0018] Referring to FIG. 2A, after performing a mask operation forforming a well region, boron ions are implanted and a thermal treatmentis carried out to thereby form a P-type well region 42 on apredetermined portion 410 of the semiconductor substrate 41 by a lateraldiffusion.

[0019] Then, after forming a field oxide layer 43 for isolation betweenneighboring unit pixels, four transistors TX, RX, DX and SX are formedon the semiconductor substrate 41. Here, each transistors TX, RX, DX andSX includes a gate oxide layer 44, a polysilicon layer 45 and a tungstensilicide layer 46. The transistors TX and RX are formed on a firstportion 400 and the transistors DX and SX are formed on the secondportion 410.

[0020] Then, after performing a mask operation for forming a photodiode47, implantation of N⁻ and P⁰ ions is carried out to thereby form an N⁻doping region 47A and a P⁰ doping region 47B which are self-aligned onone side of the transistor TX. At this time, the P-type semiconductorsubstrate 41, the N⁻ doping region 47A and the P⁰ doping region 47Bconstitute the photodiode 47 of a PNP structure.

[0021] Then, a lightly doped drain (LDD) region 49A is formed beneaththe transistors DX and SX by implanting boron ions. Spacers 50 areformed on sidewalls of the transistors TX, RX, DX and SX, and then,impurity junction layers 49B are formed by implanting N-type impurityions.

[0022] Referring to FIG. 2B, a first tetra-ethyl-ortho-silicate (TEOS)layer 51A is deposited on an entire structure by using a low pressurechemical vapor deposition (LPCVD), and a borophospho-silicate glass(BPSG) layer is deposited on the first TEOS layer 51A by using anatmospheric pressure chemical vapor deposition (APCVD). Thereafter, athermal treatment is performed to reflow the BPSG layer 51B. Here, astacked layer of the first TEOS layer 51A and the BPSG layer 51B arecalled a pre-metal dielectric (PMD) layer 52.

[0023] Then, a hydrogen containing dielectric layer 53, whose thicknessis a range of 7000 Å to 8000 Å, is formed on the PMD layer 52 by using aplasma enhanced chemical vapor deposition (PECVD). At this time, thehydrogen containing dielectric layer 53 is formed with one of SiO_(x),SiN_(x), SiO_(x)N_(y), Si₃N₄, and the like.

[0024] Then, a thermal treatment is carried out to diffuse hydrogen ionscontained in the hydrogen containing dielectric layer 53 to a surface ofthe photodiode 47. Thereafter, a dry etching or wet etching operation isperformed to remove the hydrogen containing dielectric layer 53. Here, ahydrogen content in the hydrogen containing dielectric layer 53 iscontrolled by a degree of vacuum, a temperature and an injection amountof NH₃/SiH₄ gases in the process of the PECVD.

[0025] Referring to FIG. 2C, an isotropic etching operation is performedby using a buffered oxide etchant (BOE) to thereby form a contact holeon the impurity junction region 49B. Then, Ti/Al/TiN layers 54, 55 and56 are sequentially deposited on an entire structure, and a maskoperation and an etching operation are carried out to form a first metalline 57.

[0026] Referring to FIG. 2D, a second TEOS layer 58 is deposited on thefirst metal line 57 by using the PECVD. A spin on glass (SOG) layer 59is coated two times and a planarization of the SOG layer 59 is performedthrough a thermal treatment and a plasma. Then, an interlayer insulatinglayer 60 is deposited on the SOG layer 59 by using the PECVD.

[0027] Referring to FIG. 2E, after performing a mask operation, anisotropic etching operation is carried out by using the BOE, and then,an anisotropic etching operation is carried out to thereby form acontact hole. Then, Ti/Al/TiN layers 54A, 55A and 56A are sequentiallydeposited on an entire structure, and a mask operation and an etchingoperation are carried out to form a second metal line 61. Thereafter, anoxide layer 62A and a nitride layer 62B are deposited on the secondmetal line 61 by using the PECVD.

[0028] Referring to FIG. 2F, the nitride layer 62B and the oxide layer62A are selectively etched to expose a predetermined portion of thesecond metal line 61. Then, a dyed photoresist is coated on a upperportion of the photodiode 47 and a color filter 64 is formed by using anoperation of a development. Then, a microlens photoresist layer 65 isformed and a microlens 66 is formed on the microlens photoresist layer65.

[0029] As described above, by diffusing the hydrogen ions contained inthe dielectric layer into the surface of the photodiode by using thePECVD, an unstable dangling bond is removed and a recombination ofcarriers is reduced in the surface of the photodiode, thereby reducingthe dark current of the photodiode caused by a leakage current.

[0030] Although the preferred embodiments of the invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A method for fabricating a CMOS image sensorhaving a plurality of unit pixels, comprising: a) providing asemiconductor structure having a photodiode on a semiconductorsubstrate; b) forming an insulating layer covering the semiconductorstructure including the photodiode; c) forming a dielectric layer havinghydrogen over the insulting layer; d) diffusing hydrogen ions from thedielectric layer into the photodiode; and e) removing the dielectriclayer.
 2. The method as recited in claim 1 , wherein the step of forminga dielectric layer includes forming it with a material selected from agroup consisting of silicon oxide (SiO_(x)), silicon nitride (SiN_(x)),silicon oxide nitride (SiO_(x)N_(y)) and Si₃N₄.
 3. The method as recitedin claim 2 , wherein the step of forming a dielectric layer includesplasma enhanced chemical vapor deposition (PECVD).
 4. The method asrecited in claim 1 , wherein the step of diffusing hydrogen ionsincludes thermal treatment.
 5. The method as recited in claim 1 ,wherein the step of removing the dielectric layer includes dry etchingor wet etching.
 6. The method as recited in claim 1 , wherein the stepof forming a dielectric layer includes depositing it to a thickness of7000 Å to 8000 Å.
 7. The method as recited in claim 1 , wherein the stepof forming a dielectric layer includes forming it only on an upperportion of the photodiode.